Lightwave communication systems, especially, long-haul lightwave communication systems, remain under active development worldwide. Techniques and apparatus are being reported for achieving much longer communication distances at progressively higher bit rates. In such systems, fiber nonlinearities and dispersion tend to limit the available alternatives for modulation formats which can accommodate the data speed requirements while being relatively unaffected by the nonlinearities.
Since propagation of soliton pulses in optical fiber depends on the presence of group velocity dispersion in the fiber, soliton pulse propagation has been proposed as one method for transporting lightwave information in a telecommunications system. See, for example, U.S. Pat. No. 4,558,921 which discloses a soliton-based optical fiber telecommunications system and U.S. Pat. No. 4,700,339 which discloses a wavelength division multiplexed soliton-based optical fiber telecommunications system employing periodic Raman amplification to compensate fiber loss. In order to overcome intrinsic losses in the optical fiber, non-electronic amplification elements are disposed along the telecommunication system to amplify the soliton pulses. Non-electronic amplification elements provide amplification of the signal as a photon pulse without changing it into an electron pulse. These amplification elements include doped-fiber amplifiers, semiconductor traveling wave amplifiers, Raman amplifiers, and phase coherent, continuous wave, injection amplifiers. In the '921 patent, non-electronic amplification elements are taught as providing the additional required capability of decreasing the width of the soliton pulses while simultaneously increasing their peak power.
As the length of the entire telecommunication system exceeds several hundred kilometers, it is necessary to space amplification elements apart by a predetermined distance to form an amplification chain along the length of the optical fiber. That is, a plurality of optical amplifiers are interconnected by individual lengths of optical fiber. Spacing of amplification elements continues to be a relatively inexact science for single channel systems owing perhaps to the specification by long haul telecommunication systems designers that, for cost and future system maintenance reasons, the interamplifier spacing should be on the order of 100 km. Such a specification disregards effects on system performance and quality. In wavelength division multiplexed soliton systems, the spacing of Raman amplifier pump sources was determined to be an appropriate distance which would introduce only a small velocity (wavelength) shift of the soliton pulses. For the '339 patent, the interamplifier spacing L for Raman pump sources was determined in relation to the soliton period, z.sub.0, as z.sub.0 &gt;L/4. In that patent, it was also recommended that the interamplifier spacing satisfying z.sub.0 &lt;L/16 was also recommended as desirable to overcome soliton stability problems in the vicinity of z.sub.0 .apprxeq.L/8. While these guidelines exist for Raman amplification systems, there are no clear guidelines for interamplifier spacings in lumped amplifier systems such as those systems including doped-fiber amplifiers.